In Rameau's Nephew, Diderot attacked and ridiculed the critics of the Enlightenment, but he knew from past experience that some of his enemies were sufficiently powerful to have him arrested or the work banned. Diderot had done a spell in prison in 1749 after publishing his Lettre sur les aveugles (Letter about the Blind) and his Encyclopédie had been banned in 1759. Prudence, therefore, may have dictated that he showed it only to a select few.
After the death of Diderot, the manuscript or a copy of it probably made its way to Russia. In 1765, Diderot had faced financial difficulties, and the Empress Catherine of Russia had come to his help by buying out his library. The arrangement was quite a profitable one for both parties, Diderot becoming the paid librarian of his own book collection, with the task of adding to it as he saw fit, while the Russians enjoyed the prospect of one day being in possession of one of the most selectively stocked European libraries, not to mention Diderot's papers.
An appreciative Russian reader communicated the work to Schiller, who shared it with Goethe who translated it into German in 1805.
Hegel quotes Rameau's Nephew in §522 of his Phenomenology of Spirit.